1. Field of the Invention
The present invention relates to a device able to detect acceleration through a switching operation of a switch caused by movement of a mass body due to an action of acceleration. More particularly, the invention relates to an acceleration detector for detecting a collision of a mobile unit such as a vehicle and for actuating its airbag.
2. Description of the Related Art
Generally, an acceleration detector of this kind includes a mass body that is housed movably back and forth in a case; a coil spring backwardly urging the mass body; a stationary contact provided in the case; and a moving contact provided in the mass body. The acceleration detector is arranged such that when the vehicle collides against something, the mass body forwardly moves against urging force of the coil spring, and the moving contact contacts the stationary contact to energize a circuit, thereby detecting acceleration larger than a given value.
The set sensitivity (threshold of the detected acceleration) of the acceleration detector is determined by a mass of the mass body, a spring constant and an initial load of the coil spring, and the distance between the moving contact and the stationary contact under unloaded condition.
JP 9-211023 A discloses an acceleration detector in which a mass body is slidably pierced via a through hole by a sliding shaft provided in the longitudinal direction in a case, and is backwardly urged by a coil spring wound around the sliding shaft. The patent publication further says that this conventional mass body is composed of a main-mass member and a sub-mass member each having a through hole formed therein.
The sub-mass member is provided with a truncated cone-shaped spring-holding cylinder that projects forwardly from the fringe of a central hole of a thick disk, and with a plurality of collision buffers on the front of the thick disk. The small cylinder of a front end of the main-mass member is fitted into an inner surface of the spring-holding cylinder, and the diameter of the front end of the small cylinder, projecting from the forward end of the spring-holding cylinder, is outwardly expanded to caulk the sub-mass member to the main-mass member.
Meanwhile, a plurality of contact segments of the moving contact are extended from the periphery of a thin disk, a central hole of the thin disk is engaged with the small cylinder of the main-mass member, and the thick disk is pressed against the main-mass member by the sub-mass member, which holds and fixes the thin disk between the main-mass member and the sub-mass member.
The main-mass member is usually manufactured by a zinc-die casting method or cold-forging processing of copper or brass. One example of the manufacturing processes of the zinc-die casting method is as follows. Die casting→annealing→barrel polishing→deburring→shot blasting→copper underplating→nickel plating→inner-surface burnishing for a through hole. In this way, the zinc-die casting method is usually complicated and entails a lot of processes.
JP 2001-050975 A, which says a relevant art, discloses an optical fiber acceleration sensor including a diaphragm equipped with an optical fiber coil in such a manner that the coil-expands and contracts in opposite directions each other; an acceleration detecting portion composed of a supporting base supporting the diaphragm and a weight secured on the diaphragm; and an optical component composed of an optical coupler and FRM that are connected with the optical fiber coil and output interference light by forcing light to be interfered with one another, which is input to the optical fiber coil and propagated through the same, wherein a potting resin is filled in a space of the sensor to secure the optical component.
Further, JP 11-174077 A discloses an acceleration detector including a diaphragm at the center of which a weight is provided; a base supporting the periphery of the diaphragm; and an acceleration sensor that is secured on a surface opposing the surface on which the weight of the diaphragm is provided and outputs an acceleration signal according to what extent the diaphragm is deformed, which is resulted from acceleration impressed on the weight, wherein the diaphragm is formed of plastic resin material.
Moreover, JP 11-295334 A discloses an acceleration sensor including a lead switch having an output terminal that turns on by a change of a magnetic field, and outputs a rapid deceleration detecting signal of minute electric current; a cylindrical inner housing that houses therein the lead switch; a magnet mass that is provided movably in an axial direction around the periphery of the inner housing, and inertially moves at the time of rapid deceleration to cause a magnetic field to be changed; a spring that is installed around the periphery of the inner housing and urges the magnetic mass in the direction opposite to inertial movement to control the inertial movement; an outer housing that houses therein the inner housing and the magnetic mass; and an amplifier circuit amplifying the rapid deceleration detecting signal.
In an airbag system in recent years, although at the first stage of collision, a vehicle receives small impact acceleration, it has been necessary to judge whether a collision is occurred or not at the early stage and deploy an airbag, even in a collision typified by an offset collision where high impact acceleration is generated after a fixed time has elapsed. For that purpose, the mass body, on which acceleration is impressed, should sensitively and stably slide forward along a sliding shaft against urging force of the coil spring.
However, because the main-mass member which constitutes the conventional mass body is dominated by a zinc-die casting product or a cold-forging processed product of copper or brass, slidableness of the member is deteriorated due to formation of rust-under high temperature and humidity conditions. Accordingly, there is a possibility that the required response characteristic could not be secured to collision acceleration from diagonal directions generated at the time of an offset collision. For this reason, it calls for surface treatment thereof for the purpose of improving corrosion resistance and sliding characteristic thereof. However, this treatment brings about a drawback in productivity and a cost. Particularly, surface treatment to the zinc-die casting product is extremely complicated and difficult.
Further, the number of components of an airbag system tends to increase for its expanding functionality. Therefore, a space in the substrate on which the acceleration detector is mounted is becoming relatively smaller, and there remains need for further size reduction of the mass body.
In addition, actuation timing of an airbag varies from vehicle to vehicle depending on when to be deployed it, which requires to make the sensitivity of the mass body adjustable according to a type of a vehicle.